The present invention relates to a magnetic toner and an image forming method using the same.
In image forming apparatuses such as laser printers, electrostatic copying machines, plain paper facsimiles, and their complex machines utilizing electro photographic methods, electrostatic recording methods, or electrostatic printing methods, a surface of a latent image holding member is uniformly charged by charging means, and is then exposed by exposure means such as semiconductor lasers or light-emitting diodes, to form an electrostatic latent image on the surface, and the electrostatic latent image is then developed into a toner image by developing means. The toner image is then directly transferred to a surface of a material to be printed such as paper by transfer means or is transferred to a surface of an intermediate transfer member, is then transferred again on the surface of the material to be printed such as paper, and is then fixed to the surface by fixing means, to complete a series of image forming steps.
Developing methods for developing an electrostatic latent image into a toner image are roughly divided into dry developing methods and wet developing methods. Currently, the dry developing methods have spread widely. The dry developing methods are classified into developing methods using magnetic toner having magnetic powder involved in its toner particles composed of binder resin (a magnetic one-component developing method, a magnetic two-components developing method, etc.) and developing methods using non-magnetic toner having no magnetic powder involved therein (a non-magnetic one-component developing method, a non-magnetic two-components developing method, etc.) when they are classified on the basis of the type of toner to be used.
In the magnetic one-component developing methods, the magnetic toner is supplied while forming a thin layer of the magnetic toner on a surface of a developer carrying member incorporating a fixed magnet, and the electrostatic latent image on the latent image holding member is then developed into the toner image by the thin layer of the magnetic toner. Examples of the magnetic one-component developing methods include developing methods using magnetic toner having conductive properties and developing methods called magnetic one-component jumping developing methods using magnetic toner having insulating properties. Currently, the latter magnetic one-component jumping developing methods have spread widely.
In the magnetic one-component jumping developing method, the magnetic toner is first supplied to a surface of the developer carrying member while being subjected to triboelectric charging by being passed through a clearance between the developer carrying member that is rotated and that is contained a fixed magnet therein and a magnetic blade disposed in close proximity to the developer carrying member, and is held by a magnetic force of the contained fixed magnet, to form a thin layer of the magnetic toner on the surface of the developer carrying member.
A direct current bias voltage or a bias voltage obtained by overlapping an alternating current with a direct current is applied between the latent image holding member for holding the electrostatic latent image and the developer carrying member that are opposed to each other with a clearance held there between so as not to come into contact with the formed thin layer, thereby scattering the charged magnetic toner on the surface of the latent image holding member from the thin layer, to develop the electrostatic latent image into a toner image.
In the magnetic one-component jumping developing method, the magnetic toner having insulating properties is used, so that transfer of the formed toner image on a surface of a material to be printed such as paper utilizing an electric field, which has been impossible in a case where toner having conductive properties is used, becomes possible. Further, the latent image holding member can be also prevented from being destroyed by electric leakage.
Further, the magnetic toner having insulating properties also has the following advantages:
the magnetic toner is easily charged,
the magnetic toner can be sufficiently rubbed against the developer carrying member in a state where magnetic toner is held by the magnetic force, and
the electrostatic latent image can be developed in a non-contact state with the electrostatic latent image while holding the magnetic toner by the magnetic force.
Therefore, an image superior in quality can be formed by preventing such fogging that toner adheres to a non-printed portion and a blank portion of the formed image.
In recent years, in an image forming apparatus, such two currents, that is, increase in image formation speed and miniaturization have been rapidly progressing. In a high-speed machine mainly adaptable to business applications where the increase in image formation speed is required, the resolution and the quality of a formed image are liable to decrease as the printing speed increases. In order to prevent that, it is required that the charging amount of the magnetic toner easily rises more quickly than ever before and is more stable than ever before.
On the other hand, in an intermediate- and low-speed machine designed for small offices and general homes where miniaturization is required, the turn-on and turn-off of power are frequently repeated. In order to make warming-up time after turning on power as short as possible, therefore, initial charging of the magnetic toner must be good. For the image forming apparatus, further increase in resolution and quality of the formed image, improvement in durability of the magnetic toner, improvement in stability to environmental variations, and so forth have been continuously required irrespective of the difference in the image formation speed depending on applications, for example.
In order to satisfy the requirements to stably maintain good image properties (high image density, no fogging, and superior image quality) under various temperature and humidity environments over a long time period, the following are required for the magnetic toner:
a charging amount easily rises quickly, and
a proper charging amount can be always maintained without causing deficiency in charging amount and charge-up (excessive charging) under such environments that charging is difficult, for example, high-temperature and high-humidity environments and conversely, such environments that charging is excessive, for example, low-temperature and low-humidity environments, and the proper charging amount can be maintained over a long time period.
Currently in the magnetic toner generally employed, however, the requirements have not been sufficiently satisfied in the present circumstances in the above-mentioned currents, that is, increase in image formation speed and miniaturization. The main cause is magnetic powder involved in the magnetic toner according to inventor's examination.
Generally used as the magnetic powder are currently magnetic powder in the shape of a polyhedron such as a hexahedron (a cube, a rectangular parallelepiped) that is a convex polyhedron surrounded by six quadrilaterals and an octahedron that is a convex polyhedron surrounded by eight triangles and magnetic powder in the shape of a sphere.
In the magnetic toner using the polyhedral magnetic powder, however, pointed vertexes or pointed edges between adjacent faces of the magnetic powder are exposed to surfaces of toner particles and charges are easily discharged there from, so that the charges are liable to leak. The polyhedral magnetic powder is low in fluidity and is inferior in dispersibility to binder resin. Accordingly, it is difficult to uniformly disperse the magnetic powder in the binder resin. Therefore, there easily occur variations in the dispersed state and the content of the magnetic powder among the toner particles, so that the magnetic toner also easily varies in the ease of charging, the charging amount, and so on.
In the magnetic toner using the polyhedral magnetic powder, therefore, the charging amount does not easily rise quickly, and the charging amount itself is small. As a result, image defects such as decrease in image density and occurrence of fogging are liable to occur. Further, the magnetic toner easily varies in the ease of charging and the charging amount depending on the temperature and humidity environments at the time of image formation. Particularly under environments, where charging is difficult, such as high-temperature and high-humidity environments, the above-mentioned image defects are liable to further occur.
On the other hand, the spherical magnetic powder has no pointed vertexes and edges. In the magnetic toner using the spherical magnetic powder, therefore, charges are not easily discharged from the magnetic powder exposed to the surfaces of the toner particles, so that the charges do not easily leak. The spherical magnetic powder is superior in fluidity and is also superior in dispersibility to the binder resin compared with the polyhedral magnetic powder. Accordingly, the magnetic powder is easy to uniformly disperse in the binder resin. Therefore, the magnetic powder can be made uniform in the ease of charging, the charging amount, and so on by preventing variations in the dispersed state of the magnetic powder from occurring among the toner particles.
In the magnetic toner using the spherical magnetic powder, however, charges are conversely too easily stored. Therefore, there occurs so-called charge-up in which the magnetic toner is charged in excess of not less than a predetermined charging amount in cases such as a case where the magnetic toner is repeatedly rubbed in a clearance between the developer carrying member and the magnetic blade. When the charge-up occurs, image defects represented by decrease in image density rather easily occurs.
In order to make use of both the respective advantages of the spherical magnetic powder and the polyhedral magnetic powder, therefore, magnetic powders having various particle shapes have been examined.
The following documents, for example, disclose magnetic powders each having the particle shape of a polyhedron such as a hexahedron or an octahedron, whose vertexes and edges are each subjected to so-called chamfering at a plane smaller than each of faces constituting the polyhedron.                Japanese Unexamined Patent Publication No. JP11-153882A (1999)        Japanese Unexamined Patent Publication No. JP2000-162817A        Japanese Unexamined Patent Publication No. JP2000-242029A        
In the magnetic powders disclosed in the documents, however, pointed edges still exists between the face composing the polyhedron and the small plane used for the chamfering. Charges are easily discharged from the edges. Accordingly, the charges are liable to leak from the magnetic toner, so that image defects such as decrease in image density and occurrence of fogging may occur.